Add support for mavlink message DEBUG

msg/CMakeLists.txt

@@ -47,6 +47,7 @@ set(msg_file_names
 	control_state.msg
 	cpuload.msg
 	debug_key_value.msg
+	debug_value.msg
 	differential_pressure.msg
 	distance_sensor.msg
 	ekf2_innovations.msg

msg/debug_value.msg

@@ -0,0 +1,3 @@
+uint32 timestamp_ms		# in milliseconds since system start
+int8 ind			    # index of debug variable
+float32 value			# the value to send as debug output